It has been well known to incorporate metallic pigments in coating compositions, in particular it is an established practice to use aluminum flake pigments in compositions intended for the production of so-called "glamour" finishes on automobile bodies, whereby a differential tight reflection is effected, depending on the angle at which the coated surface is viewed. Generally speaking, such coating compositions have been solvent-based, in common with the other coating compositions employed in the automobile industry. The tend is growing, however, toward the use of water-based coating compositions due to the need to improve work environments and reduce pollution.
For example, British Patent Specification No. 2073609 teaches a coating process in which a specified type of water-based base coat composition containing pigments, especially aluminum flake pigment is applied to a base surface, after which a transparent top coat composition is applied to the base coat.
However, difficulties arise in incorporating metallic pigments, particularly aluminum or aluminum alloys, into water-borne coating compositions, because the pigment tends to react with water to produce hydrogen.
This leads to problems, especially if compositions are stored in closed containers.
Ways have been proposed to minimize or prevent this "gassing effect" most involving some form of chemical treatment of the metallic pigment to render it less reactive toward the aqueous medium of the coating composition. It is known that ortho-phosphoric acid is effective for this purpose, but its presence in coating compositions leads to unacceptable poor humidity resistance in films derived from them.
Alkyl esters of phosphoric acid can overcome the problem to a limited extent, but their use involves an unsatisfactory compromise between the inhibition of gassing and a tendency toward deteriorating mechanical properties in the derived coatings, in particular poor adhesion between the base coat containing the agent and the transparent top coat.
The same is true of nonphosphorus-containing treatment agents, such as dimer acids, proposed for this purpose in British Patent Specification 2043092.
Furthermore, U.S. Pat. No, 4,621,112 corresponding to Japanese Patent Unexamined Publication No. 47771/1986! discloses the use of a reaction product of a Phosphoric acid and one or more compounds which contain in the molecule at least one epoxide group and which also contains in the molecule at least one aromatic or alicyclic group wherein one or both reactants contain an aliphatic group of more than six carbon atoms with the aliphatic group not exceeding 65% by weight. The reaction product provides antigassing without sacrificing the mechanical properties of the coatings.
In the instance of the additive, although mechanical properties are not affected, coating appearance, particularly "flop" is adversely affected. The "flop" means the visual change in brightness of the metallic aluminum flake with a change in viewing angle, that is, a change of from 90 to 180 degrees.
The greater the visual change from a light to dark appearance, the better the flop. Flop accentuates the lines and curves of a coated angle. Furthermore, a starting material of the antigassing agent in the USP is an aromatic glycidyl epoxy compound such as a bisphenol A glycidyl epoxy compound, prepared by the use of epichlorohydrin, resulting in that chlorine is brought in the gassing agent. Even in exceedingly minor amounts, the presence of chlorine adversely affects metallic pigments.
Still further, in the case when the starting material is an aromatic glycidyl compound, it would not be stable with light and oxygen, resulting in inferior weatherability.
In addition, European Patent Publication No. 0319971 corresponding to Japanese Patent Unexamined Publication No. 190765/1989! discloses a water-based coating composition containing aluminum flakes and an antigassing phosphatized acrylic polymer in which an epoxy-functional acrylic polymer is allowed to react with a phosphoric aced, or forming in situ phophatized polymer after an acrylic monomer is allowed to react with a phosphoric acid.
However, it has been found that the antigassing agent described in the EP is somewhat inferior from the viewpoint of a balanced stability of the composition, the tone of color in the coated layer, anti-strippability in the coated layer and weatherability or moisture resistance in the coated layer. Furthermore, the molecular weight of phosphatized acrylic polymer is not easy to adjust.
As a result of an intensive investigation, the inventors of this invention have now found that it is possible to solve the problems as described hereinabove by the use of the phosphatized alicyclic compound of the present invention as an antigassing agent for a water-based coating composition containing aluminum powder or flake pigments.